This invention relates in general to antifriction bearings and, more particularly, to a precision bearing assembly and a method of assembling the same to achieve minimum runout in the bearing-supported component.
In certain machinery, rotating shafts must run true, that is to say they should have as little runout as possible. Runout, of course, represents the deviation of a circular or cylindrical surface from a fixed reference point as it rotates, and could perhaps best be characterized simply as wobble. Several factors contribute to runout, and these fall into generally two categories, namely tolerances that cause bearing inner races to be slightly eccentric, when ideally they should be concentric, and radial play in the bearings which support the rotating part. The interrelationship between these two factors contribute significantly to the shaft's overall runout. Other less significant factors such as roller size irregularities also contribute to the shaft's runout.
Typical of machinery where runout must be held to an absolute minimum are the spindles of machine tools, such as lathes and milling machines, and the rolls of rolling mills. Indeed, in a rolling mill, any runout in the exterior surface of a work roll or back-up roll will detract from the quality of the sheet or other product which is produced in that mill, even when this variance may be barely perceptible.
Two types of bearings find widespread use in such machinery, those being the tapered roller bearing and the cylindrical roller bearing, for each is capable of carrying heavy radial loads. Tapered roller bearings lend themselves to adjustment in that the axial positions of the races relative to each other control the radial clearance or play and may even eliminate the radial clearance altogether. This in turn provides control of the size of the load zone, that is the number of rollers which are actually under load at any instant. Notwithstanding this capability, tapered roller bearings when used to transmit extremely heavy loads, such as in rolling mills, are usually manufactured with their tapered rollers arranged in four rows and with the tapers of adjacent rows oriented oppositely. This makes adjustments in the field difficult, and for all practical purposes these multirow bearings are adjusted at the factory through the selection of spacers. There the bearings are usually set with a slight amount of axial clearance. However, the profile of the raceway on the cone or inner race is not perfectly circular nor is its axis perfectly coincident with the axis of rotation. These imperfections cause runout. Cylindrical roller bearings, on the other hand, cannot be adjusted, and since some radial clearance must exist between the rollers and the raceways in order to assemble the bearings, cylindrical roller bearings inherently will have runout, given that the inner race cannot be ground perfectly round.
Despite the inherent runout, the cylindrical roller bearing for mill rolls affords easier control over the runout derived from manufacturing tolerances, for the raceway of the inner race, once that race is installed on the roll neck, may be ground reasonably true with respect to the axis of the roll. Typically, the roll, with the inner races of its bearings in place, is placed between centers with the roll neck later supported on steady rests. The grinding eliminates much of the runout caused by manufacturing tolerances in the roll neck and race and the expansion tolerance of the press fit, although a little runout will remain due primarily to imperfections in the grinding operation itself, for the inner race may have one or more lobes in its profile after grinding. This alternative has not been available for tapered roller bearings, primarily because their races are at different inclinations, and the inner races or cones have ribs, all of which interfere with the grinding. Furthermore, alteration of a radial dimension on a tapered raceway affects axial dimensions in the bearing and requires spacers or shims to compensate for the change in the axial dimension.